Technical Field
The present disclosure relates generally to building ventilation and, more particularly, to a ventilation system that exploits a temperature difference between a building's interior and the surrounding atmosphere.
Background Art
Using gravity for moving heating, circulating and ventilating air is a simple technique that has been a well understood and practiced for more than 100 years. Hot air is less dense than cold air and will therefore tend to rise while cooler air tends to settle. However, the force produced in this way is very slight, and is easily overcome by friction in ducts and by wind pressure around a building. A gravity ventilation system is simpler than a forced air system, requires no skilled attention, and is less expensive to install. Widespread use of gravity heating air and water systems ended mainly because such systems:                1. were difficult to install requiring large ducts and many penetrations through floor slabs etc.; and        2. their installation required experienced engineers who could assess a building's suitability for a gravity heating systems.        
Historically, the advantages of gravity heating and circulation made it particularly advantageous for houses, small school buildings, churches, hails, etc., where a heat source may be placed near the bases of a warm air duct and where air flow resistance is low. However, unseparated air ducts in a gravity ventilation system often become inefficient due to stagnation if the duct's wall becomes exposed to cooler surrounding or adjacent air that induces downdrafts within the duct which collide with rising warmer air. Gravity air ducts that allow air to circulate simultaneously in opposite directions require very large cross-sections like an air well in multistory buildings. Also, it has been thought that using gravity for ventilation is more expensive than a fan because the amount of thermal energy required to produce a significant draft or air velocity through a duct greatly exceeds the electrical energy required to power a fan. Gravity air circulation may exhibit difficulty in moving hot air into certain rooms in a building during windy weather.